1. Field of the Invention
The present invention generally relates to a polling system for use in an audio system, and particularly relates to a polling system for confirming the status of connection of a plurality of controllers to a system controller, which controllers respectively control a plurality of I/O (input and output) devices.
2. Description of Background Information
There are audio systems made up of a plurality of I/O devices (or components), and configured such that a plurality of controllers for controlling each of the I/O devices (hereinafter referred to as slave controllers) are connected, via a bus line, to a system controller for performing the management of the state of such slave controllers, so that bi-directional communications are enabled through the bus line.
In such audio systems, the system controller performs a polling operation via the bus line upon power-up of the system by the throw-in of a power current in such a manner that the status of connection of the slave controller to the bus line is checked respectively, and the result of the check is stored so that communication is performed afterwards only between the system controller and the slave controllers whose connection to the system controller has been confirmed.
Through the initial polling operation at the time of the power-up of the system, if it is judged that no slave controller is connected, it generally means that the control of communication is unnecessary. So, a procedure is performed to restrict the operation so that the control of communication would not be performed.
However, if the system is designed such that completely no control of communication is performed subsequently when it is judged that no slave controller is connected through the initial polling operation, it is not possible to know the presence of a slave controller if the slave controller is connected after the judgment of the connection.
On the other hand, in the case of the audio system mentioned above, as many as 15 I/O devices can be connected, and addresses 01.sub.H -0F.sub.H are respectively allotted to the slave controllers of the devices. Upon power-up of the system, the system controller performs the polling operation via the bus-line, so that the connection of slave controllers to the bus line is checked and the result is stored. After this operation, communication is performed only between the system controller and slave controllers whose connection to the system controller has been confirmed. This polling operation begins starts with the designation of address to the slave controller from the lowest address (01.sub.H), and the connection of the slave controller is confirmed by checking whether or not a response from the slave controller is received within a certain time period. Subsequently, the address is incremented, and the same process is repeatedly performed until the last address (0F.sub.H) is reached.
As mentioned above, with the conventional polling system the status of connection of the slave controller is checked by sequentially designating the address from the address 01.sub.H to the address 0F.sub.H. Therefore, it has been necessary to check the status of connection with respect to all addressed even if no slave controller is connected to the system controller. Thus, there has been a problem that a period is required for the polling operation.
On the other hand, this type of audio system may be, for example, mounted in a vehicle. FIGS. 1A through 1C illustrate the operation of the system under such a condition. If each of the slave controllers has a means for detecting on/off status of the power supply, the operation of the slave controller can be stopped by a drop of the power voltage as illustrated during the operation of a staring motor of the vehicle, as illustrated in FIG. 1A. In such a case, if start of the operation of one of the slave controllers is delayed as illustrated in FIG. 1C with respect to timing of initial polling operation of the system controller which is illustrated in FIG. 1B, for example due to variation in the start time from one slave controller to another, that slave controller would be regarded not existing. Afterwards, that slave controller would be considered not being connected.